1. Technical Field
The present disclosure relates to an acoustic transducer having a split membrane that converts a sound wave into electrical signals, and to a digital electronic interface circuit for an acoustic transducer.
2. Description of the Related Art
Conventionally, Electret Condenser Microphones have been used as a miniature microphone mounted on a cellular (mobile) phone. The ECM is weak against heat. On the other hand, a MEMS microphone is superior to the ECM in terms of digitalization, miniaturization, enhancement of functionality/multi-functionality, and power savings. Accordingly, at present, the MEMS microphone is becoming widespread.
The MEMS (microelectromechanical system) microphone includes a capacitor-type acoustic sensor (acoustic transducer) that detects a sound wave and converts the detected sound wave into an electrical signal (detection signal), a drive circuit that applies a voltage to the acoustic sensor, and a signal processing circuit that performs signal processing such as amplification on the detection signal from the acoustic sensor and outputs the processed detection signal to the outside. The drive circuit and the signal processing circuit are manufactured integrally with each other as an ASIC (Application Specific Integrated Circuit) by using a semiconductor manufacturing technology.
Such acoustic transducers, are known, including a micromechanical sensing structure, designed to transduce acoustic pressure waves into an electrical quantity (for example, a capacitive variation), and a reading electronics, designed to carry out suitable processing operations (amongst which amplification and filtering operations) of the electrical quantity so as to supply an electrical output signal, either analog (for example, a voltage) or digital (for example, a PDM—pulse density modulation—signal).
This electrical signal, is further processed by an electronic interface circuit, is then made available for an external electronic system, for example a microprocessor control circuit of an electronic apparatus incorporating the acoustic transducer.
The micromechanical sensing structure in general includes a mobile electrode, provided as a diaphragm or membrane, set facing a fixed electrode to provide the plates of a variable-capacitance detection capacitor. The mobile electrode is generally anchored, by means of a perimetral portion thereof, to a substrate, whilst a central portion thereof is free to move or deflect in response to the pressure exerted by incident acoustic pressure waves. The mobile electrode and the fixed electrode provide a capacitor, and the deflection of the membrane that constitutes the mobile electrode causes a variation of capacitance as a function of the acoustic signal to be detected.
Currently, a microphone can detect and output a large sound with high quality. In general, a maximum input sound pressure (dynamic range) is restricted by a total harmonic distortion (hereinafter, referred to as “THD”). This is because attempting to detect a large sound by the microphone results in generation of a harmonic distortion in an output signal, thereby leading to deterioration of sound quality. Namely, if the THD can be reduced, then the maximum input sound pressure can be increased.
However, in a general microphone, detection sensitivity for the sound wave and the THD have a trade-off relationship therebetween. Therefore, a high-sensitivity microphone has a large THD, so as to have a small maximum input sound pressure. This is because the high-sensitivity microphone tends to output a large signal and therefore is likely to cause the THD. Meanwhile, a low-sensitivity microphone causes a small THD, so as to have a large maximum input sound pressure. However, it is difficult for the low-sensitivity microphone to detect a small sound with high quality.
In order to cope with these problems, such a microphone which uses a plurality of acoustic sensors having respective different sensitivities has been studied (for example, refer to U.S. Pat. Nos. 8,223,981 and 8,233,637, U.S. Patent Application Publication 2007/0047746 (published on Mar. 1, 2007), and Japanese Unexamined Patent Publication No. 2008-245267 (published on Oct. 9, 2008)).
Each of U.S. Pat. Nos. 8,223,981 and 8,233,637 discloses a microphone including a plurality of acoustic sensors, wherein the plurality of acoustic sensors output a plurality of signals and the plurality of signals are switched or combined in response to a sound pressure. In particular, U.S. Pat. No. 8,223,981 discloses a microphone including a high-sensitivity acoustic sensor whose detectable sound pressure level (SPL) ranges from 20 dB to 110 dB and a low-sensitivity acoustic sensor whose detectable sound pressure level ranges from 50 dB to 140 dB, wherein the microphone uses the high-sensitivity acoustic sensor and the low-sensitivity acoustic sensor in a switching manner so as to achieve a detectable sound pressure level ranging from 20 dB to 140 dB. Moreover, each of Japanese Unexamined Patent Publication No. 2008-245267 and U.S. Patent Application Publication No. 2007/0047746 discloses a configuration including a plurality of acoustic sensors independently provided on a single chip.
However, according to the above configuration described in each of Japanese Unexamined Patent Publication No. 2008-245267 and U.S. Patent Application Publication No. 2007/0047746, the acoustic sensors are formed independently of one another, and therefore variation and mismatching in their acoustic characteristics occurs. Here, the expression “variation in the acoustic characteristics” refers to a difference between the chips with regard to the acoustic characteristics of the acoustic sensor. The expression “mismatching in the acoustic characteristics” refers to a difference between the plurality of acoustic sensors in a single chip with regard to the acoustic characteristics.
Specifically, the acoustic sensors have thin films warped in respective different manners, so that variations in the detection sensitivity occur between the chips independently. As a result, a large variation between the chips occurs in the difference between the detection sensitivities among the acoustic sensors. Further, the acoustic sensors have their respective back chambers and vent holes. Since acoustic characteristics such as frequency characteristics and phases are affected by the back chamber and the vent hole, mismatching in the acoustic characteristics occurs in the chip.
As mentioned, the electrical performance of the acoustic transducer depends on the mechanical characteristics of the sensing detection structure, and moreover on the configuration of the associated, front and rear, acoustic chambers, i.e., of the chambers facing a respective, front or rear, face of the membrane, and traversed in use by the pressure waves that impinge upon the membrane and that move away therefrom.
There are numerous applications in which detection of acoustic-pressure waves with a wide dynamic range are used, i.e., the possibility of detecting signals with a high sound-pressure level (SPL), while maintaining high values of the signal-to-noise ratio (SNR), and signals with a low sound-pressure level with a high sensitivity.
Basically, a frequently important design rule is to optimize the compromise between obtaining a wide dynamic range in detection of the acoustic-pressure waves and obtaining a low signal-to-noise ratio.
U.S. Pat. No. 6,271,780 discloses, in this connection, a solution for increasing the dynamic range in an acoustic system, comprising an analog-to-digital converter (ADC), designed to receive an analog detection signal from an acoustic transducer. This solution envisages subjecting the analog input signal, in parallel, to two signal-processing paths, which have a first, analog, portion and a second, digital, portion, and each of which has a respective amplification and gain factor so as to adapt, respectively, to signals with a low, or a high, acoustic pressure level. The two digital signals at output from the two processing paths are recombined to supply a resulting output signal. Prior to the operation of recombination, the two signals undergo an operation of equalization to take into account differences of gain, offset, and phase generated by the previous operations of signal processing, in part of an analog type, and thus prevent distortion of the resulting output signal.
This solution is not free from problems, due mainly to the complexity of the processing chain, to a relevant sensitivity to noise and oscillations of the input signal, and to a reduced configurability.
In general, it is thus certainly felt to provide an improved solution for extending the dynamic range in the detection of acoustic-pressure waves via an acoustic transducer.